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While cnosam's answer is completely correct, I don't know if it really solves your confusion. The key point is that, when a photon is emitted, it knows nothing about the current size of the Universe. It is emitted at a very specific wavelength given by quantum mechanics, not by cosmology. Traveling through expanding space subsequently increases its ...


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Note You should clarify your statement from "...a charged particle cannot gain energy from a magnetic field..." to "...a charged particle cannot gain energy from a static magnetic field..." There is nothing wrong with energy transfer from time-varying magnetic fields. Background If the spatial gradient in the magnetic field is slow enough such that the ...



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